Ariane 5 exploded because one number didn't fit in 16 bits

transcript

This rocket exploded because one number didn't fit. And it had a backup computer. That crashed too.

4 June 1996. The first Ariane 5 lifts off. Two computers track where it's pointing. One active, one on standby. The standby one hits an error. It shuts itself down. About 0.05 seconds later, the active one does the same. About 40 seconds in, at 3,700 metres, the rocket breaks up.

The villain? A value called horizontal bias. 64 bits wide. The code squeezed it into a 16-bit integer. Ariane 5 flies faster sideways than Ariane 4. The number overflowed.

Here's the catch. Both computers ran the same software. The rule said: on any error, shut the processor down. So the same input killed both. One after the other. The flight computer read error data as flight data. Nozzles swung hard.

Engineers had found seven variables whose conversion could fail like this. They protected four. Three were left bare. BH was one. Ariane 5's trajectory was never in the requirements. No test ran it.

The worst part? That code only mattered before lift-off. It was an Ariane 4 leftover. It kept running anyway.

Backups save you from random failures. Not from your own bugs. Two copies of the same code fail on the same input.

Full story in the article.